This invention relates to a power transmission shaft for transmitting power with both ends of the shaft connected to joints such as constant-velocity universal joints.
Among transmission shafts for transmitting power with both ends connected to joints, there are ones in which the joint connecting portions are integrally formed at both ends and which are formed hollow over the entire length thereof. As such integral, hollow type power transmission shafts, ones in which one or both of the joint connecting portions are spline portions inserted into the joints are often used. For example, as power transmission shafts for power transmission mechanisms of vehicles, ones in which the joint connecting portions on both sides are spline portions inserted in the inner rings of constant-velocity universal joints are often used.
Such integral, hollow type power transmission shafts have an advantage that it is possible to achieve equivalent static strength while being lighter in weight than solid ones. As means for strengthening this type of power transmission shaft, induction hardening from outer peripheries and work-hardening means by reducing diameters are often employed. There are also ones in which the intermediate portion of the shaft is strengthened by work-hardening, and the joint connecting portions at both ends thereof are strengthened by induction hardening (JP patent publication 2001-208037).
This integral hollow type power transmission shaft, in which at least one of the joint connecting portions is a spline portion, differs from solid ones in the balance between the static strength and the fatigue strength. Thus, if it is designed so as to obtain static strength equivalent to that of a solid type while reducing its weight, it will be inferior in fatigue strength to a solid one. Fatigue cracks tend to occur with the rising portions of the splines as starting points.
An object of this invention is to provide an integral hollow type power transmission shaft that is superior in the balance between the static strength and the fatigue strength.